DALLAS — A rare 165-year-old map of Texas that has been in the same family for generations is expected to sell for more than $150,000 at a Saturday auction.

The 1849 document, considered the first official map of Texas after it became a state in 1845, will be offered in Dallas by Heritage Auctions.

“This is important because it shows Texas at that fledging point where it’s just becoming a new member of a fairly young nation,” said Joe Fay, Heritage’s manager of rare books.

The map was one of several historical family items 66-year-old Patrick Martin, a retired architect, brought to an appraisal event Heritage held last spring in Birmingham, Ala., near his home. He had no idea of its value, but realized he had something special when he saw Fay’s face light up.

“When the eyes fell on the signature, it was a ‘eureka’ moment,” Fay said. “It was that: ‘Wait a minute. This is brown ink. This would be black in any subsequent revision.’ So what we have here is the real thing.”

Fay said the map, a lithograph, was hand-signed by Jacob De Cordova, a land agent who commissioned an employee of the Texas General Land Office to create the map. The map measures about 32 by 35 inches and features counties in various colors.

Fay said subsequent editions are more plentiful, but the 1849 first edition is rare. He said only a handful exist today, with a couple privately owned and a couple others in institutions.

Martin said when his grandfather died in the late 1950s and the Virginia farm that had been in the family since the 1750s was being sold, his father, almost as an afterthought, loaded up several boxes filled with historical items, including the map.

As a kid, Martin loved reading through those documents and eventually, he came across the map. “After I knew the map was there, I would open the map occasionally and look at the things and show it to friends. My parents never had any problem with it. Of course, we didn’t have any idea of the value of the map,” he said.

“I was fascinated by it for several reasons. It had Indian settlements and it had forts on it,” he said.

And after inheriting it as an adult, he continued to show it to friends, including one recent Thanksgiving where he laid it out on the table. “In retrospect, knowing what it is worth, that was a dangerous venture, with all that grease and wine around,” he said with a laugh.

As far as he can tell, the map dates back to either his great-great grandfather, Nicholas Martin, or Martin’s son, Hudson. He said Nicholas Martin was a colonel in the Virginia militia that fought in Texas during the Mexican-American War and Hudson Martin, a Virginia attorney, helped settlers secure land grants in Texas.

Mark Lambert, deputy commissioner for archives and records at the Texas General Land Office, said De Cordova’s map got the seal of approval from state officials and is considered the first official map after Texas became a state. “It’s certainly one of the most important Texas maps,” he said.

He said the De Cordova’s 1849 map is the most popular reproduction sold as part of the land office’s Save Texas History program, which raises funds to preserve their collection of documents and maps by selling reproductions of their holdings. And for those who might not have the funds to buy one of the few originals, the land office’s reproduction costs only $20.